1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) apparatus and method for performing image reconstruction, using data collected when a body portion, such as the diaphragm, is positioned in a preset allowable area.
2. Description of the Related Art
The heart, for example, moves in accordance with, for example, breathing. As an imaging method employed to perform imaging of such a moving target section, there is a known movement compensation method in which data collected when the target section is positioned within a preset allowable area is utilized for reconstructing the target section. In the movement compensation method, a magnetic resonance signal (navigator echo signal) that reflects movement of the target section is acquired from an imaging subject to detect the position of the target section.
However, when the movement compensation method is employed, not all data collected can be utilized for reconstruction, and the required imaging time is lengthened. For instance, in the case where the number of imaging slices is 70, the number of lines in the direction of phase encoding is 128, and data corresponding to 8960 lines (=70×128) is needed. Assuming that data corresponding to 20 lines can be acquired by data collection during the period of one heartbeat, the period corresponding to 448 heartbeats (= 8960/20) are required to collect data corresponding to 8969 lines. If one heartbeat requires one second, the necessary minimum imaging period is 448 seconds=about 7.5 min. However, if the probability of employment of data during the above compensation operation is 50%, the actually required imaging period is about 15 min., that is twice the minimum imaging period.
When the imaging period is long, the state of breathing of a subject, i.e., the depth of breathing, may change during imaging. The depth of breathing changes when, for example, a subject falls asleep. In this case, the position of a target section is greatly changed, and may fall outside an allowable area for a long time. This further reduces the probability of employment of collected data for reconstruction, which further lengthens the required imaging period. At worst, the target section may always fall outside the allowable area. In this case, reconstruction cannot be realized, and hence the imaging operation cannot be finished.
To avoid this, a method for correcting a positional allowable area to increase data collection efficiency has been proposed (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2004-202043).
In this method, however, data concerning different sections of the target section is collected before and after the correction of the allowable area, since the region, in which data is actually collected, is not spatially moved. FIG. 10 shows a state in which the position of a target section relative to a slicing position is changed before and after the correction of the allowable area. Since reconstruction is performed using the data thus collected, the resultant image is blurred by the influence of the movement of the target section, regardless of compensation.
There is a method for dealing with such a disadvantage. In this method, the position of slicing is moved in accordance with the movement of a target section. By combining this method and the above-described allowable area correcting method, the relationship between the slicing position and the position of the target section can be maintained substantially constant. However, in this case, the slicing position is greatly moved in the Z-axis direction as shown in FIG. 11.
In heart imaging, in particular, in coronary artery imaging, it is essential to suppress the occurrence of signals from fat tissue around the arteries to enhance the contrast. Suppression of fat tissue signal levels is indispensable. Further, in general, a pulse sequence using steady state free precession (SSFP) that provides excellent contrast of heart muscle and blood is utilized as an imaging sequence. It is well known that this pulse sequence is very sensitive to static magnetic field uniformity. There is another imaging method, such as an echo planar imaging (EPI), in which the static magnetic field uniformity of an imaging section significantly influences the quality of the resultant image.
In the above-described movement compensation method, however, the slicing position is moved in the static magnetic field during imaging, which means that the slicing position cannot be kept at a position at which the static magnetic field is uniform. Accordingly, if the imaging method sensitive to static magnetic field uniformity is combined with the above-mentioned movement compensation method, the degree of suppression of the occurrence of fat tissue signals may be reduced, and a reduction in image quality, such as an increase in banding artifacts in the SSFP method, may be involved.